This invention relates generally to the production of small diameter monofilament fibers from low modulus polymeric materials and to yarns, fabrics, furs and other products made therefrom.
More specifically, this invention relates to fibers having a combination of physical properties which impart a unique degree of softness and flexibility to the products made therefrom.
Historically, man-made fibers have been engineered so that the physical properties of such fibers are about the same as textile fibers found in nature, for example, cotton or wool. Natural textile fibers are generally thin, having a diameter less than about 2 mils and a high elastic modulus, for example, a modulus greater than about 200,000 psi. Thus, synthetic fibers are thin and have a high modulus. For example, a typical commercially-available, polyethylene monofilament having a tensile strength of about 28,500 psi displays an elastic modulus of about 340,000 psi. Such thin, high modulus fibers have a stiffness parameter generally ranging between about 1.times.10.sup.-5 and about 1.times.10.sup.-8 lb-in.sup.2. In general, any fiber having a stiffness parameter within this range will feel soft and pliant. Because conventional fibers have a relatively high elastic modulus, usually well above 200,000 psi, they must have a relatively low moment of inertia. Otherwise they would feel too stiff.
Elastic modulus, designated as E.sub.f, is determined by measuring the initial slope of the stress-strain curve derived according to ASTM standard method No. D2256-69. Strain measurements are corrected for gauge length variations by the method described in an article entitled "A Method for Determining Tensile Strains and Elastic Modulus of Metallic Filaments", ASM Transactions Quarterly, Vol. 60, No. 4, December 1967, pp. 726-27.
The moment of inertia, designated I.sub.f, of a fiber is a function of its cross-sectional area. Under normal loading conditions, fibers bend about a neutral axis where the moment of inertia will be a minimum value. The moment of inertia about this neutral axis is calculated using the following equation: EQU I.sub.f =.intg.y.sup.2 dA
where dA is any incremental area of the fiber's cross-section and y is the distance any such incremental area is from the neutral axis.
For fibers with a uniform circular cross-sectional configuration, the moment of inertia (I.sub.f) may be calculated by the following formula: EQU I.sub.f =.pi.d.sup.4 /64
where d is the fiber diameter. Specific equations for calculating the moments of inertia of fibers having a cross-sectional configuration other than circular are given in a paper presented at the 47th annual meeting of the ASTM, Vol. 44, (1944).
The stiffness parameter of a fiber, designated K.sub.f, is a general indicator of the feel, or hand, of a fabric made from that fiber. This stiffness parameter is the product of the elastic modulus of the fiber and the area moment of inertia of the fiber: EQU K.sub.f =E.sub.f .times.I.sub.f
When considering the hand of any fiber, one must take into account the specific textile construction in which the hand is being judged. In a fabric of pile construction, for example, the fiber acts under loads like an upright column wherein the load to affect unit strain in bending is defined by the formula: EQU F=E.sub.f I.sub.f /I.sup.2
where I is the pile height. Normally, pile height is established by styling considerations. Thus, the stiffness parameter, K.sub.f, is a basic measure of the softness or degree of flexibility of a fabric. In general terms, one may compare the hand of different fabrics by comparing the stiffness parameter of the fibers provided that each fiber has a uniform cross-section and is composed of the same material throughout.